What is Solder Dewetting?

Proper soldering techniques are essential to creating electronic devices that function properly. In the electronics industry when a PCB is manufactured, the immediate next step is to solder all the electrical components like resistors, capacitors, and many other components to the PCB traces. However, sometimes the process doesn’t go as planned and which eventually leads to soldering issues and poor electrical joints.

This is a comprehensive article that discusses the causes of Solder Dewetting, and problems that arise due to this phenomenon. The article also contains information on how you can prevent a Solder Dewetting situation by being cautious, and well-prepared.

A representation of a very basic solder dewetting

What is Solder Dewetting?

Solder dewetting is a surface phenomenon that occurs due to the reduction of surface energy between the solder and the substrate. It is a process where the solder material loses its wetting ability and adhesion to the underlying substrate.

In the context of printed circuit boards (PCBs), solder dewetting is a process where the solder material on the PCB surface loses its adhesion to the metal pad, resulting in the formation of small spherical balls or irregular-shaped regions of the bare copper surface.

During the dewetting process, the surface energy of the solder and the substrate material becomes imbalanced, leading to the formation of surface tension gradients that drive the solder to retract from the substrate surface. This results in the formation of isolated regions of the bare substrate surface, which can compromise the mechanical stability and electrical connectivity of the solder joint.

Causes of Solder Dewetting in PCBs

Solder Dewetting is a defect that can hinder the reliability and efficiency of a PCB. Here are some of the possible causes of the same:

1. One of the most common causes of Soldering problems like Cold solder, and dewetting, in general, is surface contamination.  If the PCB surface is not properly cleaned and or its surface is contaminated by substances like oil, grease, or flux residues, the surface tension of the solder may decrease, causing the solder to pull away from the surface of the PCB. Contamination can also prevent the formation of a good intermetallic bond between the solder and the copper pad, leading to incomplete wetting.
2. Another cause of solder dewetting is the oxidation of the PCB surface. When the copper pad is exposed to air, it can form a layer of oxide that inhibits the wetting action of the solder. This is more common with lead-free solders, as they require higher temperatures, which can increase the rate of oxide formation.
3. Solderability issues can lead to Solder Dewetting as well. If the copper pad is not properly plated or coated, or if the plating is damaged during the PCB fabrication process, the solder may not wet the surface properly. Additionally, if the solder paste is too old or has not been stored properly, it may have reduced solderability, leading to incomplete wetting.
4. Finally, poor design practices can also contribute to solder dewetting. For example, if the size of the copper pad is too small, or if the design does not allow for proper solder flow, the solder may not wet the surface properly. Similarly, if the solder mask is not properly defined, or if the stencil design is not optimized, it can lead to insufficient solder paste deposition, which can result in incomplete wetting.

There are several factors that can cause solder dewetting in PCBs. PCB assemblers should take appropriate steps to minimize these factors to ensure that high-quality solder joints are achieved.

Effects of Solder Dewetting on PCBs

The effects of solder dewetting on PCBs can be significant and detrimental to the performance and reliability of the circuit. Here are some of the effects:

1. Reduced electrical conductivity: Solder dewetting can create voids or gaps in the solder joint, which can reduce the electrical conductivity of the joint. This can lead to intermittent connections, increased resistance, and even complete failure of the circuit.
2. Increased mechanical stress: Solder joints that have undergone dewetting can become weak and brittle. This can increase the mechanical stress on the joint, leading to cracking, fracturing, and other forms of physical damage.
3. Decreased reliability: Solder joints that have undergone dewetting are less reliable and more prone to failure than properly bonded joints. This can reduce the lifespan of the PCB and increase the likelihood of equipment failure.
4. Poor thermal performance: The gaps created by solder dewetting can also reduce the thermal conductivity of the joint, which can lead to increased temperatures and decreased performance of the circuit.
5. Difficulty in repair: Solder dewetting can make it challenging to repair the circuit, as it can be challenging to reflow the solder onto the affected areas. This can increase the repair time and cost, as well as the risk of further damage to the PCB.

In conclusion, solder dewetting is a critical issue that can significantly impact the reliability and performance of PCBs.

How to Prevent Solder Dewetting?

We have clearly seen the effects of Solder Dewetting and how harmful can it be for electronic devices and electrical components connected to a PCB. Let’s have a look at some of the ways we can prevent Solder Dewetting:

The image shows a clear distinction between a proper and improper soldering

1. Proper PCB design: The first step in preventing solder dewetting is to design the PCB properly. Make sure that the pads and traces are designed to minimize the thermal mismatch between the PCB and the components being soldered. Use proper spacing between the pads and traces to avoid excessive heat buildup during soldering.
2. Cleanliness: Contaminants like oil, grease, and dust can lead to poor wetting and dewetting. It is important to keep the PCB and components clean and free of contaminants. Use a suitable cleaning solvent and a lint-free cloth to clean the PCB and components before soldering.
3. Flux: Use an appropriate flux to promote good wetting of the solder on the PCB and components. The flux helps to remove any oxides on the surface of the metals and promotes the formation of strong bonds between the solder and the metals.
4. Soldering technique: Use the proper soldering technique to prevent solder dewetting. Use a soldering iron or a reflow oven that can provide uniform heating across the entire PCB. Avoid overheating the PCB or components, as this can lead to thermal mismatch and cause solder dewetting.
5. Solder alloy selection: Choose a suitable solder alloy that is compatible with the metals being soldered. The choice of solder alloy can affect the wetting and dewetting characteristics of the solder. For instance, lead-free solders are more prone to solder dewetting than leaded solders.
6. Preheating: Preheating the PCB and components before soldering can help to reduce thermal mismatch and prevent solder dewetting. Use a preheating oven or a hot plate to raise the temperature of the PCB and components to a level that is close to the melting point of the solder.

Conclusion

In conclusion, solder dewetting is a surface phenomenon that occurs due to the reduction of surface energy between the solder and the substrate, which can cause significant damage to the reliability and performance of printed circuit boards (PCBs). Contamination, oxidation, solderability issues, and poor design practices are some of the possible causes of solder dewetting in PCBs. The effects of solder dewetting can lead to reduced electrical conductivity, increased mechanical stress, decreased reliability, poor thermal performance, and difficulty in repair.

To prevent solder dewetting, PCB designers should ensure proper design practices, while PCB assemblers should take appropriate steps to minimize factors that lead to contamination, oxidation, and solderability issues. Proper cleaning and handling of solder paste, and the proper storage of PCBs can also prevent solder dewetting. Ultimately, proper soldering techniques and precautions are crucial to ensure high-quality and reliable solder joints in electronic devices.